The present invention relates to a condensation-growth particle scrubber in which submicron particles are grown or enlarged in size by injecting steam into a low temperature saturated gas stream carrying the submicron particles. The injection of steam causes supersaturation of the gas stream. Condensation forms on the small particles so that they grow in size and can be removed by conventional particle removal equipment.
Pollution control devices that have been developed for removing particulate and gaseous contaminants from combustion flue gas streams and from diesel exhaust have included filters, electrostatic precipitators, scrubbers, cyclones, and mist removal devices such as those sold under the trademark DEMISTER by the Otto H. York Co., Inc. of Parsippany, N.J. Most of these devices will remove particles that are greater than one micron in transverse dimension, and while some devices, such as filters, will remove smaller particles, the efficiency becomes quite low, or the pressure drop becomes quite high, as the smaller particles are removed. Further, baghouse filters cannot be used with high temperature gas streams or with moisture laden flows because the fabric will be destroyed or become clogged.
Electrostatic precipitators are used quite widely to remove particles by first imparting an electrical charge to the particles, and then removing the particles in an electric field. Costs of operation are quite high, and electrostatic precipitators are not compatible with certain types of dusts, such as those that have high resistivity. Particle reentrainment, poor gas flow, complexity of control systems and like are problems encountered when using electrostatic precipitators. These precipitators do not remove gaseous contaminants.
Scrubbers are often used to remove particulate and gaseous contaminants from combustion flue gas streams, and have the advantages of simultaneously absorbing gases and removing particulate matter. They can be used in high temperature, high moisture, and high dust environments, with a low initial capital cost. The removal efficiency of scrubbers depends heavily on the size of the particles, and most scrubbers are not capable of removing particles smaller than one-half a micron.
Some scrubbers use steam or compressed air to atomize water in the flue gas stream to cause agglomeration with the atomized water droplets under intense turbulent flow mixing conditions. These scrubbers have relatively high efficiency for removing submicron particles, but also consume high amounts of energy. One such device is referred to as the "hydrosonic" scrubber. These scrubbers are limited to small scale installations or specialized applications, such as those for incineration of hazardous, toxic, or radioactive wastes.
Cyclones are normally used to remove particles larger than a few microns in diameter, and mist eliminators are used primarily for removing liquid particles. High efficiency cyclones and mist eliminators are capable of removing particles as small as one micron, but they generally have a high pressure drop and high energy consumption for particles that small and smaller. This makes operation costs high since operation is inefficient.
In the patented prior art, condensational growth of particles has been known. Hausberg et al. U.S. Pat. Nos. 3,773,472 and 3,906,078, as well as Richards et al. U.S. Pat. No. 4,345,916 disclose adiabatic cooling to achieve vapor supersaturation, and using that as a way of causing particle growth to permit removal of the particles from the aerosol.
Hardison U.S. Pat. No. 3,755,990 mixes a hot gaseous effluent having a high percentage of submicron particles with a hot saturated gas stream, which is then subjected to rapid cooling, such as by diluting with cool air, so as to cause condensation of water on the particles.
Carson et al. U.S. Pat. No. 4,272,499, Martin et al. U.S. Pat. No. 3,852,409, and Ewan et al. U.S. Pat. Nos. 3,912,469 and 4,141,701, use processes which include passing a carrier gas through a turbulent free jet emitted from a subsonic nozzle and containing a large number of small high velocity liquid droplets atomized by steam, or other gases, to promote droplet growth by agglomeration with particles and other droplets. The droplets then can be removed by a specially designed cyclone. Particle collection in these devices is accomplished by agglomeration, rather than condensation of steam on the particles.
The present invention involves a process that reduces overall expense of operation, and maintains high efficiency.